A sapphire substrate having micro unevenness is known as a crystal growth substrate adopted for light-emitting diodes (see, for example, Non-Patent Literature 1).
Non-Patent Literature 1 describes a nano-patterned sapphire substrate (NPSS) as a sapphire substrate having micro unevenness. Non-Patent Literature 1 further describes an epitaxial substrate including the NPSS and AlN grown on the NPSS. Patterns on a surface of the NPSS are a plurality of concave triangle cones formed by etching a sapphire crystal. The surface of the NPSS includes inner surfaces of the plurality of concave triangle cones and surfaces (planar parts) of unetched portions of the sapphire crystal. Non-Patent Literature 1 describes that growing AlN on the NPSS enables the dislocation density in the AlN to be reduced. However, the epitaxial substrate described in Non-Patent Literature 1 still has a high dislocation density. Thus, for example, to increase the efficiency of ultraviolet light-emitting diodes, further reducing dislocations in the epitaxial substrate is desirable.
In the field of epitaxial substrates, further reducing the dislocation density in an MN layer is desirable to improve crystallinity.